Qfyilyi

When I learned, what the anode and the cathode are, there were two definitions corresponding to each other:

1. Cathode is, where electrons enter in the system.


2. Anode comes from the greek anodos meaning "going upwards", that is where current enters the system (that is the dirrection opposing to the flow of the electrons)

Now when I tried to understand the reverse biasedness of a photodiode, I struggled half an hour until I realized, that most of the sources I could find, refers the N-doped end of the diode a cathode, even if the current is now reverse.

Did I miss something? Why is that? Is there another interpretation for these notions?

There is an unfortunate discrepency in the history of the words.

In electrochemical devices such as batteries and electrolysis tanks anode and cathode are determined by the direction of ion/electron movement within the cell. Electrons and negative ions move through the electrolyte toward the anode while positive ions move toward the cathode. This means that when a battery is discharging vs. charging, the anode and cathode switch places. Technically an open circuit battery has no anode or cathode as no charges are moving.

In semiconductor diodes anode and cathode refer to the doping. N-doped is always the cathode and P-doped is always the anode regardless of the direction of current flow.

One more bit of confusing terminology: anions are species attracted to anodes, and therefore negatively charged. Cations are positive ions.

This is a diode:

Image From uv201.com

It is a vacuum tube with two terminals (not counting the heater). Much like a triode is a three terminal vacuum tube.

The cathode emits electrons by thermionic emission, and those electrons are only attracted to the anode if it is at a higher voltage, meaning it rectifies the current flowing through it.

Fifty years later a semiconductor device with the same purpose was invented.